Such a construction of a semiconductor device is held together by the fixing plastic package molding compound, which at the same time embeds the semiconductor chip and is firmly anchored on the leadframe. The semiconductor chip is additionally fixed in its position on the leadframe by means of an adhesive layer, the adhesive layer ensuring that the semiconductor chip is neither displaced during the application of the plastic package molding compound nor lifts off from the leadframe. At the same time, the adhesive layer under the semiconductor chip acts as a stress equalizing layer when thermal stresses occur.
However, the actual equalizing effect of the adhesive layer under the semiconductor chip is not fully realized, since it is enclosed on all sides by the rigid plastic package molding compound. Consequently, the adhesive layer, which is restricted to the surface of the semiconductor chip, is also laterally fixed and cannot completely equalize the difference in stress between the leadframe and the semiconductor chip that is caused by the different coefficients of expansion. Consequently, in temperature cycle tests virtually the entire stress between the semiconductor chip and the circuit substrate is passed on to the external contacts of the semiconductor device, which can lead to considerable failures.
In spite of the equalization of thermal stresses by the adhesive layer between the semiconductor chip and the leadframe, microcracks can also occur in the case of such a construction of a semiconductor device, in particular at the boundary layer between the plastic package molding compound and the upper sides of the leadframe. It has previously been attempted to prevent such microcracks, which in an extreme case can lead to delamination, by strengthening the anchorages between the two materials. For this purpose, porous ceramic or metallic coatings have been developed for the upper side of the leadframe and deposited on the leadframe in order to improve the anchorage with the plastic package molding compound. Nevertheless, it is observed that the stronger anchorage has the effect that the formation of microcracks is shifted into regions above the anchoring layer but cannot be completely suppressed.
For these and other reasons there is a need for the present invention.